1. Field of the Invention
The present invention relates to a wavelength multiplexing and demutiplexing device.
2. Description of the Related Art
Dielectric multi-layer films are generally used as the anti-reflection films coated on the surfaces of spectacle lenses or as the television color-separation filters coated on glass substrates. With the miniaturizing trend of devices, in color separation filters used in liquid crystal projectors and cameras, etc. or laser detection mirrors used in DVD (Digital Versatile Disk) devices, those models have come to be required which are structured such that a dielectric multilayer film is held between two prism-shaped glass substrates and a light is incident at an angle on the dielectric multilayer film. In the communications sector, to cope with the huge increase in traffic on the Internet, the introduction of multiple-wavelength transmission technology has been pushed forward and, above all, there is requirement for filters, including a dielectric multilayer film serving as an edge filter or a band-pass filter formed on the glass substrate to separate light of different wavelengths.
In optical communication, by combining 3-terminal modules in cascade, it becomes possible to combine (multiplex) or separate (de-multiplex) light of various wavelengths, but because modules are required as many as the number of waves combined or separated, and the likely results are increases of device cost, area required, and installation cost. As is disclosed in JP-A-8-82711 and “Optical Engineering” by Yoichi Fujii, Agune Shofusha, 1993, pp168-169, modules have been proposed which incorporate a plurality of band-pass filters and edge filters in a single module to thereby combine or separate multiple wavelengths; however, since the beam-splitting angle is small, a problem arises that if one is going to mount a laser for transmission and a diode for reception, it is necessary to secure a long optical path, which leads to increases in device size and installation cost. If one tries to reduce the size of the device, it is necessary to use a laser/diode array, which results in cost increase. If an attempt is made to achieve downsizing without cost increase, it is required to increase the beam-splitting angle, in which case, however, a problem arises that a deviation becomes large between P-wave and S-wave of the output light, which results in a deterioration of the beam multiplexing/demultiplexing characteristics.
To improve the deterioration problem of characteristics which depends on the p-polarized light and the s-polarized light at a high deviation angle, or a high incident angle, in other words, to improve the problem that a considerable deviation occurs in the amplitude wavelength characteristics of the outgoing light depending on the polarizing direction of the incident light, Si is used for the high refractive index layer of a dielectric multilayer filter in JP-A-2000-162413. If a substance with high refractive index, such as Si, Ge, ZnS, ZnSe, for example, is used for the high refractive index layer, the characteristic differences in the amplitude characteristics can be improved, but as disclosed in JP-A-2000-162413, when TiO2 or SiO2 is used for the low refractive-index layer, if left as it stands for a long time at a high temperature of 85° C. and a high relative humidity of 85%, oxygen of TiO2 or SiO2 diffuses to the side of the high refractive-index layer, causing a decrease in the refractive index of the Si or Ge layer, a wavelength shift by a rise of the refractive index of the low refractive-index layer, and changes in the optical characteristics. A problem with ZnS or ZnSe is that because of the low adhesion to SiO2 or TiO2, the ZnS or ZnSe tends to break away.
If the medium of incidence is air with a refractive index of 1, the characteristic differences due to different polarizing directions can be reduced. However, with the optical parts of late, in which the degree of element integration has become very high owing to the trend of miniaturization, the filters are used often in direct contact with other optical parts, such as fiber capillary tubing, prisms or waveguides. In such a case, when air is used as the incident medium, it is necessary to employ the air sandwich structure. If the air sandwich structure is used, it follows therefore that an anti-reflection film is formed to suppress changes in the amplitude due to multiple reflections at the bonding surfaces. Because this anti-reflection film is made optimal to the air with a refractive index of 1, if a resin or the like is allowed to infiltrate to the light transmission surface in a bonding process, the transmission characteristics will deteriorate, and therefore it is required to adopt such a bonding structure as to prohibit an unwanted intrusion of resin, which is another factor of cost increase. Instead of this, if mere resin bonding is used, a decrease in process yield will result.
There is another method which adjusts the planes of polarization of the incident light to eliminate the differences in characteristics due to the different planes of polarization. If any one of the polarized light is extracted by using a polarizer, this results in a decrease in the amount of light. If, after splitting the incident light into an s-polarized light and a p-polarized light and the polarization states are made uniform by converting the s-polarized light into the p-polarized light or the p-polarized light into the s-polarized light, this method makes it necessary to provide a part to convert the planes of polarization, thus increasing the size of the device and raising the cost.